Recently, a social problem has developed involving various kinds of noises such as traffic noise by cars, etc., and noises from factories and the like. These noises are of a wide range of frequencies, as from several tens of hertz or several thousands of hertz.
Examples of conventional sound-absorbing materials are as follows: products comprising an aggregate such as sand, gravel, etc., and a binder such as a synthetic resin, an asphalt and the like, and products of glass fibers. However, these conventional sound-absorbing materials are not useful for absorption of noises of wide compass ranging from low to high frequencies, because some have sound-absorbing ability only for high frequencies and others only for low frequencies.